Elecraft's K3
transceiver, when equipped with the optional KXV3 Interface Option board,
provides an isolated, wideband, sample of its 8.215 MHz IF. The sample is
brought out to a BNC connector on the K3's rear panel. The sample is from the
K3's main receiver.

If you prefer not to read the detail,
you may skip directly to my recommendations.

The illustration below provides an overview
of how the IF sample is derived. Since the sample is taken before the K3's
narrowband "roofing" crystal filters, the sample's bandwidth is limited only by
the K3's input bandpass filters.

So far, so good. However, Elecraft's designers and I
disagree on the desired IF sample "transfer gain" of the sample port. The term
"transfer gain" is appropriate, rather than just "gain" because the IF sample is
at a different frequency than the input signal. Based upon typical
specifications for military and commercial receivers, I believe the transfer
gain should be approximately 0 dB. In other words when the K3 is set to 7 MHz, 1
uV signal at 7 MHz into the antenna connector should yield close to 1 uV
at 8215 KHz out of the IF sample port, measured into a 50 ohm instrument. I
wouldn't be too concerned with a dB or two one way or the other, but the target
should be 1:1, or 0 dB transfer gain.

In fact, it's not uncommon for military and commercial
receivers to have a significant positive transfer gain at the IF output sample,
thereby allowing an external panadapter with relatively poor sensitivity to be
used. The theory is apparently that it's easier to reduce the panadapter's
sensitivity with the gain control or via an attenuator than to have to add an
outboard amplifier.

Watkins Johnson, for example, provides the following
specification for the WJ-8718 series HF receivers:

The WJ-8718's IF output (at 455 KHz) is after the receiver's
AGC and filters, rendering it most useful for outboard demodulators, likely its
intended purpose.

Cubic's R-2411 receiver has both a
wideband and narrowband IF output port. The wideband port, suitable for use with
a panadapter, has approximately 10 dB transfer gain. The 455 KHz sample, like
the WJ-8718, is post filtering and post AGC, and is intended for output
demodulators.

Watkins Johnson's WJ-861X series receivers (for my impression
of this receiver, click here)
provides purchasers with an option. The stock 21.4 MHz wideband IF output has a
transfer gain of +15 dB, but the WBO option provides an IF sample with AGC. The
stock wideband IF output would be more suitable to use with an external
panadapter or spectrum analyzer, whilst the WBO option is best suited for an
external demodulator.

I measured the K3's transfer gain at 7 MHz. The K3's input
signal is from an HP8657A signal generator set to -60.0 dbm. The 8215 KHz
output level is measured with an Advantest R3463 spectrum analyzer. Checking
the HP8657A signal generator's output with the R3463 spectrum analyzer
showed -59.0 dBm. The data below is based upon input signal level of -59.0 dBm.

Preamp

Attenuator

8215 KHz Output Signal

Transfer Gain/Loss

Off

Off

-76.7

-17.7

On

Off

-66.3

-7.3

Off

On

-86.2

-27.2

On

On

-75.6

-16.6

In the most normal operating conditions, therefore, the
8215 KHz IF sample is between 7 and 17 dB below the level of the signal at the
antenna port, not the desired 0 dB to positive gain.

Is this a problem? My measurements suggest the answer is
"yes."

I looked at two potential panadapters; a Softrock and my
Z90. (Telepostinc's LP-PAN has a built-in amplifier to overcome the transfer
gain problem.)

For the tests discussed below, I used the Softrock 6.2
described at my pages linked above. The Softrock receiver is packaged in a
shielded enclosure, with 600:600 ohm wideband isolation transformers on both
audio output channels. An E-MU 0202 USB-connected sound card running at 192 ks/s
is used in these tests and a Dell laptop computer, with a dual core Pentium
processor and 4 GB RAM. The signal source is an HP8657A synthesized signal
generator, with a GPS disciplined 10 MHz external time base. Flex Radio's 1.12.1
software is used in these measurements. The K3 is set to the 7 MHz band, with
the preamplifier off.

As a starting point, we note that a -100 dBm (2.2 µV)signal at 8215 KHz fed directly into the Softrock's
antenna port is easily observable, being 15 dB or so above the noise level.

Setting the 8657A signal generator to 7 MHz and connecting it
to the K3 (and connecting the Softrock to the K3's IF output port) shows that
the -100 dBm signal is barely visible above the noise. The K3's 17 dB transfer
loss causes a significant loss in usable sensitivity.

Adding an external amplifier (a
Z10000-U) between the K3's IF output port
and the Softrock significantly improves the displayed signal. The Z10000-U
employed is set for 10 db gain and has a 50 ohm input impedance, obtained by
adding a 49.9 surface mount resistor to the Z10000-U's input pad. If intended to
be used only with the K3, the 49.9 ohm resistor could be dispensed with and
the Z10000-U's gain reduced to the 6 - 8 dB range. (As discussed below, the K3's
IF output port is not 50 ohms.)

I designed the Z90 to integrate with an Elecraft K2 using a
Z10000-K2 buffer amplifier to provide an IF sample at net 0 dB transfer gain.
Accordingly, I expected additional gain to be necessary when using a Z90 with a
K3.

With similar test conditions, except the input signal is
-96 dBm, not -100 dBm, a Z90 connected directly to the K3 shows a barely
perceptible trace.

First, we see background
noise, which was below the first image's sensitivity. The -96 dBm signal is seen
centered on the Z90's display. albeit with some noise fluctuations.

Of course, turning the K3's preamplifier on
increases the IF sample signal by approximately 10 dB. However, operating the K3
with the preamplifier on may or may not be desirable, depending upon the
frequency band and signal levels in general.

In particular, Elecraft notes that Additional isolation
circuitry may also be required.

Isolation can work in either direction; to keep signals
that might leak from the panadapter back into the K3 or to keep signals other
than the 8215 KHz IF from getting into the panadapter's input.

We'll first look at isolation from the outside world into
the K3 via the IF sample port.

To evaluate the port isolation, I used the following test
setup. Set the K3 to 7.000 MHz, AGC turned off, preamplifier and attenuator
turned off. CW mode, bandwidth set for 600 Hz, passband centered on 600 Hz..
Connect an HP8657A signal generator to the K3's antenna port and a true RMS
voltmeter to the Line Out audio output (I used an HP 3400A). Set the signal
generator to 7.000 MHz, touch up the K3's tuning if necessary to center the
signal in the passband and increase the signal generator's output (starting at
-140 dBm) until the signal plus noise as observed on the true RMS voltmeter
increases to +20 dB over the reading observed with the signal generator off.

Remove the signal generator from the K3's antenna port and
connect a 50 ohm termination on the antenna port. Set the signal generator to
8215 KHz and connect it to the K3's IF output port. Increase the signal
generator level until the signal is easily audible. Using the K3's spot or
frequency tuning display option, adjust the signal generator's frequency if
necessary to be center tuned, as with the reference signal. Adjust the signal
generator's level until the same true RMS voltmeter reading is observed as with
the direct input case.

The IF sample port isolation is the difference in signal
generator level between the two connection methods.

I found the isolation to be 67.8 dB. I don't believe my
measurement equipment and technique is accurate to 0.1 dB, so I'll call it 68 dB
in round numbers.

Is this enough isolation? It's difficult to impossible to
answer this in the abstract. If we look at a couple of specific examples,
however, we can sense whether it is or is not.

First, the Z90 is designed so that the strongest leakage
signal out of the signal input port is less than -80 dBm. When combined with 68
dB internal isolation, any unwanted signal is going to be at a level below
normal measurement ability.

The Softrock Lite receiver is a different story, however.

The image below shows the local oscillator leakage from a
40 meter Softrock Lite receiver at around -39 dBm. The Softrock Lite used as a
K3 panadapter has a 8192 KHz local oscillator, with leakage levels similar to
those of the 40 meter unit.

When combined with 68 dB isolation, the resultant signal
will be -107 dBm. If the Softrock Lite were operating at 8215 KHz, this level of
signal would be quite objectionable in the K3's IF. However, the Softrock's
local oscillator is approximately 23 KHz offset at 8192 KHz, which is outside
even the K3's widest FM roofing filter.

There is always a concern when unwanted signals are
injected back into a receiver's IF chain. In this case, however, I believe the
risk of problem is relatively low, considering the signal levels involved.

The main caveat to this statement is that the measured 68
dB isolation is between the IF sample port and the receiver's IF chain. The
isolation between the IF sample port and other portions of the K3's circuitry
are not known and are not, in general, easily measured.

If a Z10000-U buffer amplifier is installed to pick up the
K3's gain shortfall, of course, the extra isolation provided by the Z10000-U
will knock even the Softrock Lite's signal leakage down into the noise level.

The second
component of isolation is knowing what signals in addition to the 8215 KHz IF
sample come out of the IF sample port.

To determine this, I connected an Advantest spectrum
analyzer to the IF sample port and observed emitted signals over the range 0 -
50 MHz, with the K3 set to the bottom edge of the band for all amateur bands
between 1.8 and 50 MHz.

The image below is typical of my measurements. The K3's
local oscillator produces a reasonably strong leakage signal, typically in the
-50 to -60 dBm range. The local oscillator is 8.215 MHz above the tuned
frequency, except on 50 MHz where it is below the tuned frequency.

Rather than present a series of these images, one for each
band, I've made a composite image by overlaying all measurements onto a single
plot.

The result is quite impressive. Whether these local
oscillator leakage signals will cause your panadapter a problem is difficult to
say. We do know, for example, that the Softrock type receivers have known
spurious responses at odd harmonics, e.g., at 3 x 8215 KHz, at 5 x 8215 KHz,
etc. These responses have reduced sensitivity by a factor of 20 * log(n) where n
is the harmonic number, and the Softrock has some bandpass filtering in the
front end.

When running the same 1.8 to 50 MHz analysis, all the
plots with the filter in place look the same—nothing to be seen. All the local
oscillator leakage is reduced below the spectrum analyzer's noise floor, as
shown in the plot below.

If the K3 is
equipped with the KRX3, second receiver option, another possible leakage or
isolation concern is from the IF sample port back to the KRX3's IF.

To measure this, I used the same approach as with the main
receiver, but listening and measuring the KRX3's audio output for comparison
signal levels.

First, the good news. With an extremely strong signal, 0
dBm, at 8215 KHz, pumped into the K3's IF output sample port, there is no trace
of direct 8215 IF leakage into the KRX3's IF chain. This is excellent isolation.

The not-so-good news is that I found a new spurious path
with about 90 dB isolation. I believe, however, that this may be more of a
theoretical issue than a real one for most operators.

This may be a bit complicated, so please bear with me.

The figure below shows a simplified view of the K3 with
the second receiver. The black arrows show the normal signal flow when the main
receiver is tuned to 7000 KHz and the sub-receiver to 7010 KHz.

If a strong signal at 8205 KHz is injected into the IF
sample port, there is sufficient leakage backward through the K3's post-mixer
amplifier, balanced mixer and antenna coupler to produce a clearly audible
signal into the sub receiver. This signal path is shown in red in the image
below.

I've used these specific frequencies as an example, but in
general for any frequency that the main and sub receivers are tuned
to, it is possible to generate an on-frequency spurious signal into the sub
receiver by appropriate selection of an 8 MHz range signal injected back into
the IF sample port.

The observed spurious signal results from the signal leaking
into the K3 via the IF sample port mixing with the main receiver's local
oscillator, as shown in the table below

Thus, for a given frequency injected into the IF sample
port, as you tune the main receiver, the spurious frequency changes. In the case
of an 8205 KHz signal, for example, the spurious will move with the main
receiver tuning, always being 10 KHz above the main receiver dial frequency.
(These comments ignore for simplicity the fact that the K3's IF frequency is not
precisely 8215 KHz.)

If the K3 is not equipped with the KRX3 sub-receiver, the
same mixing phenomena still occurs, of course.

Either with or without the KRX3, the spurious will be
emitted from the K3's antenna port associated with the main receiver.

The spectrum analyzer plot below shows the signal leaking
out of the K3's antenna port when a strong 8204 KHz signal is injected into the
K3's IF sample port. The transfer coupling is -89 dB in this case.

As should be apparent from the simplified block diagram, switching the K3's
preamp on will add another isolation stage to the reverse signal path and
should considerably reduce the spurious signal out of the K3's antenna port.

As the image below, taken with the same test
conditions but with the K3's preamp on, the spurious signal is reduced another
16 dB.

So, is this a real problem
or one that is of only theoretical concern? In all but highly unusual
circumstances, I believe it's unlikely to be a problem for the following
reasons:

In order to be detectible in the KRX3 sub receiver,
the signal injected into the IF sample port must be -20 dBm or stronger. The
worst case signal leaking out of a Softrock receiver used as a panadapter is
around -40 dBm. This provides 20 dB margin which should be adequate.

The prevailing coupling path seems to be through the
K3's receiving antenna splitter. If the problem is observed, switching the
KRX3 to the external receiver input port drops the spurious below the noise
level with a 0 dBm level signal injected into the IF sample port.

In order for the signal to be heard, three
frequencies have to coincide, which reduces the statistical chance of seeing
the problem.

The most likely way this spurious problem will occur would
be for a strong signal from a nearby transmitter such as multi-band
simultaneous operations, to be coupled into the IF port through, for example, a
long poorly shielded cable connected to a panadapter, or the panadapter itself
being unshielded. If actually observed, correcting the shielding problem should
ameliorate the interference. Or, an 8215 KHz bandpass filter could be added to
the K3's IF sample output port.

Looking
at the K3 circuit fragment below, we see why the IF sample has a significant
amplitude drop.

Q10, a J310 JFET, is configured as a source follower. Its
gate is driven by a voltage divider comprising R89 and R8. This voltage divider
presents Q10's gate with a 15 dB loss compared with the signal at J77, pin 1.

Secondly, JFETs are notoriously poor as source followers
when feeding a low impedance load. An FET's output impedance is 1/gm,
where gm is determined at the FET's quiescent point. (gm
is also known as gfs in some data sheets) The J310 has a quoted gm as
between 8000 and 18000 µS according to Fairchild Semiconductor's data sheet, but
even that spread of numbers is only valid for a particular idle current.

If we take the midpoint of the data sheet numbers gm
is 12,000 microsiemens so 1/gm is about 83 ohms. Connected to a 50
ohm load, therefore, an additional loss of 8 dB is seen. (I measured the output
impedance of my K3's IF sample port as 74 ohms, quite close to the 83 ohm
midpoint estimate.)

Hence, the total loss between Pin 1, J77 and the signal
level seen in a 50 ohm measuring instrument can be estimated as 23 dB.

We can now estimate the overall transfer gain and see
how close it comes to our measured 17.7 dB loss.

Stage

Gain or Loss (dB)

Input Bandpass Filter

-1

Misc diode switches (total)

-1

First Mixer

-6

Post Mixer Amp (Q8&Q9)

17

Source Follower

-23

Total Estimated Transfer Gain or Loss

-14

We're about 4 dB short of the measured loss. (The
block diagram notes 5 dB loss in the noise blanker. However, the IF sample
point is connected ahead of the noise blanker attenuator.) I've estimated the
mixer loss at 6 dB in the absence of data in the Operator's Manual. I'm puzzled
as to source of the extra 4 db loss.

It should be possible to increase Q10's output by
altering the R89/R8 voltage divider, preferably by increasing R8. If R8 is, for
example, increased to 4.7K, the voltage divider loss decreases from 15 db to 6
dB, picking up 9 dB gain.

If you make this modification, I suggest measuring the
voltage across the new R9. Q10's drain current is set by a combination of the
voltage drop across R9 and the individual characteristics of the J310 part in
your K3. FETs have a notoriously wide part-to-part spread in parameters and it's
possible that the J310 device in your K3 has parameters sufficiently far from
the mean that Q10's power dissipation limits will be reached or exceeded. The
surface mount J310 has a maximum power dissipation rating of 350 mw, and for
reliability a safe operating value is 200 mw or so. Q10's current can be easily
determined by measuring the voltage drop across R9. If changed to 49.9 ohms,
Q10's drain current Id is 1000*Vs/50 (in milliamperes) where Vs is the
voltage measured from ground to Q10's source pin. To calculate the power
dissipated in Q10, measure its drain voltage. The power is then (Vd-Vs)*Id in
milliwatts, where Id is in milliamperes.

An example.
After replacing R9 with a 49.9 ohm resistor, the following voltage readings are
found: Vs = 2 volts, Vd = 12 volts. The idle current through Q10 is thus 2 /
49.9 = 40 mA. The power dissipated in Q10 is (12-2) * 40 = 400 milliwatts. This
exceeds the J310's maximum permissible power dissipation and would not be a good
design practice.

To increase the IF sample signal level consider an
external
Z10000-U broadband buffer amplifier.
Build the Z10000-U for 6 dB to 8 dB gain, with the normal high Z input.

An alternative is to modify R8 and R9 as described
above. If this approach is taken, verify that Q10's maximum power
dissipation limit is not exceeded, after being detrated according to good
engineering practices.

The Z10000-U will also reduce signal leakage back
into the K3 into the noise level when using a Softrock

These steps are not necessary with an LP-PAN
panadapter as it has a built-in isolation amplifier.

If you wish to eliminate the K3's local oscillator
output signals, consider a Z10010-K3
bandpass filter. If you use both a Z10000-U and Z10010-K3 filter, the
Z10000-U's gain should be increased by 3 to 4 dB to compensate for the
filter's loss.

Depending upon your requirements for display
sensitivity, it may not be necessary to use an amplifier or filter at all or
to modify the K3's IF sample components.

The Z90 or Z91 has an
8215 KHz stock IF frequency selection. In running tests recently, however, it
appears undesirable to use the stock 8215 KHz IF frequency selection with the
K3. The reason is that the Z90's sweep oscillator exhibits greater spurious
responses than desirable at 8215 KHz.

I recommend defining a new "optional" frequency of 24215
KHz and using that with the K3. The Z90's IF is 8.000 MHz and when set for
24215 KHz the Z90's sweep oscillator is centered at 16215 KHz. This frequency
also provides a response at 8215, i.e., 16215 - 8215 = 8000 KHz. When running at
16215 KHz, the Z90's sweep oscillator exhibits far fewer spurious responses.

In addition, in certain bands, the Z90's dynamic range can
be extended if a Z10010-K3 bandpass filter is used. Some amateur bands have
strong shortwave broadcast signals a few hundred KHz above or below the amateur
band. The Z10010-K3 bandpass filter will reduce the strength of these signals
when entering the Z90's input stage. (By design, the Z90 has no frequency
selective filters on the input stage.)